Packaging of the vehicle

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Moreover, this performance was strongly helped by tire heaters application, devices banned in official F.S.A.E. competitions. Furthermore, like It’s possible to notice by Picture 4.1-2, aerodynamic set-up of the car was strongly modified compared to that exploited during events. It’s possible underline that rear wing was removed in order to reduce the aerodynamic drag and save weight. However, front wing is maintained, that’s necessary to balance the longitudinal weight transfer, providing load on the front axle where a couple of traction motors operate.

Concluding topic about gear ratio, it’s necessary remark that the chosen value is enough to guarantee a good traction in autocross-endurance tests. Moreover a suitable final speed is guarantee.

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Plot 4.2-1: Position of the motor into the monocoque.

Dissertation is going to be developed around the placement of motors. In order to simplify the topic, next statements are going to be focused on one half of the car, with one only motor and one only transmission. In accordance with Plot 4.1-1, reference system adopted on the vehicle is depicted by Picture 4.2-2, x,y,z-axis and relative plans which are mentioned in the following dissertation coincide with those displayed on the picture. In addition, It’s necessary to clarify that output shaft of the motor is intended to be parallel to x-axis.

By point of view of vehicle dynamics, it’s clear that motor, due to its own mass, have to be placed in the lowest position inside the chassis In order to keep as low as possible the centre of gravity of the vehicle. Some important information are available in order to define accurately placement. By Ref.[1], minimum ride height of the car, “ground clearance”, it’s imposed: value is set to 1,0 [inch.]

which corresponds to 25,4 [mm]. Thickness of floor of the monocoque depends mainly by thickness of chosen honeycomb, like it was revealed at Chapter 1.2. So, overall thickness can be roughly esteemed to be around 25 [mm].

Lowest point internal to the monocoque can be easily calculated along z-axis with the sum of ground clearance and floor thickness. As consequence, position of motor output shaft along z-axis is well determined, as depicted in the Plot 4.2-1. Position of motor output shaft along y-axis have to be chosen as close as possible to the mid plane of the car, in order to limit the inertia around the roll axe of the vehicle, Ref.[2] Ref.[3]. Main boundary is dimension of the front flange, for this reason position of motor is well determined. Longitudinal position, along x-axis, is a bit more complex to determinate. By virtue of that, It’s necessary to perform some evaluations and packaging studies that are going to be deeply described in the following Chapters. Congruously with this level of the study, position of motor output shaft is clearly determined in vertical and longitudinal position (yz-axis).

Picture 4.2-2: XYZ axis convention for a vehicle (https://www.motor-talk.de/).

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Plot 4.2-2: Position of rim and half-shaft.

Ideally, transmission axle or “half-shaft”, needs to operate parallel to the axis of the wheel that drags.

This condition guarantee top efficiency of tripod joints around the complete field of rear suspension operation. Preliminary design of the vehicle, depicted in Plot 4.2-2, establish that rear wheels were bounded to y=0 plan, in parallel position. It means that the half-shaft works parallel to y-axis and that the axis of the half-shaft coincides with the axis of the rear wheel. Wheelbase of the car is known, therefore coordinates of the transmission output shaft on x-axis and z-axis, are clearly determined.

Basic design boundaries are fixed, It’ s now possible to evaluate some different solutions in order to find the best configuration for motor/transmission layout. That is going to be the main topic of following chapters.

Picture 4.2-3: Example of tripod joint employed on F.S.A.E. car.

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4.2.1. Transversal with internal transmission.

Picture 4.2-4: Transversal motors with internal transmission.

That is the configuration adopted by S.C.12e project and observed in chapters 1.5.1, 1.5.2 and 1.5.6. Transversal layout means that motors are positioned perpendicular to the road axis, parallel to y-axis of the vehicle. Obviously, input and output shafts needs to be parallel among them. In this configuration, gears of the transmission are all cylindrical and probably spur in a project oriented on a motorsport application. By virtue of that, actual represents the best solution by point of view of transmission mechanical efficiency, Ref.[6].

Main drawback of this solution is represented by distance between motors and rear axle. Such dimension is controlled by the size of gears. As revealed previously by Chapter 3.1, the main target in a race car design is the performance. For this reason, dimension of gears needs to be kept as compact as possible, in order to save weight and limit inertia. By other hand, dimension of motor front flange is quite large. Compact gears and large flange causes issues in the offset between output shaft of motor and rear axle of the vehicle. In first approximation calculus, this dimension tends to be shorter than needed. That causes a serious lack of space, with important difficulties in the housing of transmission components. Indeed the space in which floating half-shaft needs to be free may be too tight. That leads to probability of dangerous interferences between motor and transmission axle.

Short offset between output shaft of motor and rear axle of the vehicle doesn’t lead to packaging problems only. It leads to an important displacement along x-axis of the centre of gravity of the vehicle. In this way, intended value of front/rear weight distribution is difficult to achieve.

Such issues, in S.C.12e project, were overcome by the adoption of a drive chain transmission system. Anyway, as explained at Chapter 1.4.1, this solution is not suitable by point of view of mechanical efficiency, which is one of the main design targets of S.C.R. Moreover, drive chain transmission system may cause issues regarding the recovery of kinetic energy. Alternatively, it would be suitable evaluate a transmission provided of drop gears, like which shown at Chapter

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1.5.6. Anyway, this solution too is lacking by point of view of mechanical efficiency. Moreover, may suffer an excessive weight.

Another great issue of this solution derives from aerodynamic aspects. Motors would be housed in low rear position. This layout doesn’t leave enough space to install a rear under-body diffuser which features proper dimensions and related efficacy.

In accordance with dissertations dealt in this chapter, it’s possible to establish that transversal layout with internal transmissions may be the better solution by point of view of mechanical efficiency. Anyway it’s quite improper by point of view of packaging, vehicle dynamics and aerodynamic efficacy. Drawbacks look to be more than benefits.

4.2.2. Transversal with external transmission.

Picture 4.2-5: Transversal motors with external transmission.

House transmission on external position may be the proper solution to solve issues about lacking of space. In this way, interferences between half-shaft and motor are surely avoided. In addition, mass of transmission axle is significantly reduced decreasing inertia of the entire transmission system.

Mechanical efficiency still represents the strong point of this solution, especially in case of spur gears application.

By point of view of weight distribution situation is still worsen, because mass of motors is located in the same position. While mass of gear-box is shifted towards external of the car. In this way, inertia of the vehicle around roll axis is significantly increased, with a probable deterioration of handling performance.

Issues related to aerodynamic are identical and still unsolved. For reasons explained above, it’s possible to conclude that this type of solution results improper for the studied application.

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4.2.3. Longitudinal.

Picture 4.2-6: Longitudinal motors.

This layout features motors installed in position parallel to the road axis. The consequence is that input and output shafts need to be orthogonal. This kind of configuration can be appreciated with some examples described at Chapters 1.5.3, 1.5.4, 1.5.7 and 1.5.8. A configuration based on longitudinal motors is surely worse in terms of mechanical efficiency. Power through orthogonal axes is usually transmitted by bevel gears which feature a lower efficiency compared to cylindrical spur gears.

Picture 4.2-7: Transversal vs. Longitudinal motors fitted with monocoque and suspension.

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Loss of efficiency is quantified between 5% and 6%, Ref.[6]. Anyway efficiency is comparable with a drop gear solution and it’s surely higher than a drive chain configuration.

Moreover this solution is worse in terms of transmission overall mass too. By experience, It’s possible to notice that, on equal terms of tooth face width, bevel gear is weaker than cylindrical one. At the same time, bevel gears need to be installed on angular contact bearings that are usually bulky compared to radials bearing comparable loads. Angular contact bearings are installed to oppose to axial component of force generated by bevels and absent in spur gears. Moreover It’s necessary remind that gear-box cases needs to be reinforced in order to bear axial loads. By virtue of explained issues, transmissions including bevel gears are often heavier than full spur gears transmissions.

Anyway by point of view of weight distribution, longitudinal configuration ensures some advantages. First of all, It’s necessary to remember that all evaluated solutions features motors positioned on the floor of the chassis. For this reason, distribution of weights along z-axis is mainly the same. The real benefit of longitudinal solution is the installation of motors in forwarded position, closer to the centre of gravity of the vehicle. That feature allows to maintain intended value of front/rear weight distribution. Furthermore, masses of motors are closer to the mean plan of the car, y=0. That reduces significantly inertia of the vehicle around the roll axis, with important benefits by handling point of view.

About packaging issues, it’s convenient analyze Picture 4.2-7. It’s extremely clear that a transversal layout, suitable for a tubular space frame, is not adequate for the tapered shape of the monocoque. But the most significant aspect is that dimensions of motors and gear-box cause issues of integration with frame and suspensions. Furthermore affordability of electric contact boxes the maybe compromised.

By aerodynamic point of view, motors shifted forward offer more space to design a suitable diffuser. This item generates important benefits for overall performances of a formula car.

At the end of this deep evaluation it’s clear that longitudinal configuration is the best solution suitable in S.C.R. project. Issues related to mechanical efficiency and weight are widely overcame by benefits of layout, weight distribution and aerodynamic.